Quantum Mechanics and Physical Reality

N. Bohr

Editor’s Note

In May 1935, Einstein, Podolsky and Rosen published a landmark paper arguing that quantum mechanics must be considered incomplete. They had described a thought experiment in which the values of some physical variables could be known before they were measured, implying that some aspect of physical reality must correspond to those variables, which quantum mechanics however failed to describe. Here Niels Bohr argues that their argument was flawed owing to an essential ambiguity in quantum mechanics, which demands that physical reality cannot be considered independently of the conditions that define an experiment. So began an argument over the completeness of quantum theory, and its apparent “non-local” character (the possibility of action at a distance), which continues to this day.　　中文

IN a recent article by A. Einstein, B. Podolsky and N. Rosen, which appeared in the Physical Review of May 15, and was reviewed in Nature of June 22, the question of the completeness of quantum mechanical description has been discussed on the basis of a “criterion of physical reality”, which the authors formulate as follows: “If, without in any way disturbing a system, we can predict with certainty the value of a physical quantity, then there exists an element of physical reality corresponding to this physical quantity”.　　中文

Since, as the authors show, it is always possible in quantum theory, just as in classical theory, to predict the value of any variable involved in the description of a mechanical system from measurements performed on other systems, which have only temporarily been in interaction with the system under investigation; and since in contrast to classical mechanics it is never possible in quantum mechanics to assign definite values to both of two conjugate variables, the authors conclude from their criterion that quantum mechanical description of physical reality is incomplete.　　中文

I should like to point out, however, that the named criterion contains an essential ambiguity when it is applied to problems of quantum mechanics. It is true that in the measurements under consideration any direct mechanical interaction of the system and the measuring agencies is excluded, but a closer examination reveals that the procedure of measurements has an essential influence on the conditions on which the very definition of the physical quantities in question rests. Since these conditions must be considered as an inherent element of any phenomenon to which the term “physical reality” can be unambiguously applied, the conclusion of the above-mentioned authors would not appear to be justified. A fuller development of this argument will be given in an article to be published shortly in the Physical Review.　　中文

(136, 65; 1935)

N. Bohr: Institute of Theoretical Physics, Copenhagen, June 29.